A flexible printed circuit board is a flexible wiring board with a conductor pattern formed on the surface of an insulating resin film. Flexible printed circuit boards have become common in recent years as means of achieving increased miniaturization and higher density in electronic devices. Most flexible printed circuit boards employ aromatic polyimides as resin films.
Flexible printed circuit boards employing aromatic polyimides have conventionally been manufactured by a process in which, generally, a copper foil is bonded to a polyimide film as the insulating layer using an adhesive such as an epoxy resin or acrylic resin. With flexible printed circuit boards obtained by this process, the level of properties such as heat resistance, chemical resistance, flame retardance, electrical characteristics and adhesiveness depend on the properties of the adhesive used, and therefore it has not been possible to satisfactorily exhibit the excellent properties of aromatic polyimides.
Methods of heat sealing polyimide films to metal foils using thermoplastic polyimides as adhesives have therefore been proposed (Patent documents 1-3). There have also been proposed methods of direct cast coating of a metal foil such as a copper foil with a solution of a polyamic acid (polyimide precursor) having a thermal expansion coefficient equivalent to that of the metal foil, and removing the solvent to produce a high molecular weight product (hereinafter referred to as direct coating methods) (Patent documents 4, 5). There are also known methods of forming metal layers by vapor deposition or sputtering onto polyimide films (Patent document 6).    [Patent document 1] Japanese Unexamined Patent Publication HEI No. 3-104185    [Patent document 2] Japanese Unexamined Patent Publication No. 2005-44880    [Patent document 3] Japanese Unexamined Patent Publication No. 2005-96251    [Patent document 4] Japanese Unexamined Patent Publication SHO No. 58-190093    [Patent document 5] Japanese Unexamined Patent Publication SHO No. 63-69634    [Patent document 6] Japanese Patent Publication No. 3447070